Optical fibre sensor system where variations in temperature of optical source is minimized or equal

ABSTRACT

An optical fibre sensor system comprising a transmitter, a receiver and a sensor interconnected by an optical link. An optical source at the transmitter transmits to the sensor via the optical link two optical signals at different wavelengths. The sensor transmits to the receiver via the optical link measurement and reference signals, the measurement signal being derived by modulating the intensity of one of the two optical signals in dependence upon a parameter monitored by the sensor and the reference signal having an intensity which has a predetermined relationship with the intensity of the other optical signal. The receiver compares the intensities of the measurement and reference signals to derive a measure of the monitored parameter. The two wavelengths are selected such that changes in the intensities of the optical signals resulting from variations in the temperature of the optical source are minimised or are of substantially equal magnitude for both the optical signals.

The present invention relates to an optical fibre sensor system in whicha parameter is monitored by a sensor providing a measurement signaloutput in the form of an optical signal the intensity of which is afunction of the monitored parameter.

Dual wavelength intensity modulation sensors are known. One wavelengthis used for the transmission of a measurement signal and the otherwavelength is used for the transmission of a reference signal. Themeasurement signal intensity is modulated in accordance with themonitored parameter whereas the reference signal intensity isindependent of the monitored parameter. Ideally such a system should beconfigured such that any extraneous intensity effects proportionallyaffect both the measurement and the reference signal. If this is thecase a ratio of the two optical signals provides a fully referencedoutput independent of such extraneous intensity effects

In one known system (K. Kyuma, S. Tai, T. Sawabe and M. Nunoshita,"Fiber optic instrument for temperature measurement", IEEE J. QuantumElectron., Vol. QE-18, pp. 676-679, 1982) the optical absorption of athin semiconductor material is temperature dependent at one wavelengthand almost transparent at another. Optical emissions from two LEDsoperating at these two wavelengths were coupled together and transmittedthrough the system. The output signal of the temperature dependent lightwave normalised by that of the reference light and the ratio wastherefore dependent on the absorption spectrum of the semiconductormaterial.

A somewhat similar referencing technique again using two LEDs emittingat different wavelengths has been proposed (P. R. Wallace, E. S. R.Sikora and A. J. Walkden, "Engineering optical fibre sensors for processcontrol", GEC J. of Research Vol. 2, pp. 129-134, 1984). In this systemthe individual wavelength optical signals are reflected back both beforeand after a fibre microband sensor. The optical signal reflected beforethe sensor functions as the reference signal whilst the optical signalmodulated by the microbending sensor provides the measurement signal.

A problem with all of the devices referred to above is that they employtwo separate LED sources to produce the reference and measurementsignals. Consequently unequal spectral shift, or output powerfluctuations, in the LED sources can create problems within the sensorsystem and hence invalidate the referencing. In particular, it has beenfound that the normalised output power of LEDs is highly temperaturedependent and thus unless compensation for optical source temperature isprovided the true relationship between the measurement and referencesignals cannot be predicted and thus sensor system calibration isinaccurate.

It is an object of the present invention to provide an optical fibresensor system which obviates or mitigates the problems outlined above.

According to the present invention there is provided an optical fibresensor system comprising a transmitter, a receiver and a sensorinterconnected by an optical link, wherein the transmitter comprises anoptical source arranged to transmit to the sensor via the optical linktwo optical signals at different wavelengths, the sensor is arranged totransmit to the receiver via the optical link measurement and referencesignals, the measurement signal being derived by modulating theintensity of one of the two optical signals in dependence upon aparameter monitored by the sensor and the reference signal having anintensity which has a predetermined relationship with the intensity ofthe other optical signal, and the receiver is arranged to compare theintensities of the measurement and reference signals to derive a measureof the monitored parameter, characterised in that the two wavelengthsare selected such that changes in the intensities of the optical signalsresulting from variations in the temperature of the optical source areminimised or are of substantially equal magnitude for both the opticalsignals.

The transmitter may comprise a single LED, in which case filters areprovided to select from the LED output the two wavelengths or a pair ofLEDs each associated with a respective filter to provide the twowavelengths. The filters may be provided at the transmitter, the sensor,the receiver or at any part of the optical link therebetween.

Preferably the optical link comprises one or more optical fibres. Thesensor may be of any convenient form, for example a movement sensorcomprising a mirror the position of which modulates the measurementsignal and a fixed optical component which returns a reference signal ofconstant intensity to the receiver.

In the case of a single LED optical source, a plot of the relativeintensity versus wavelength of the source over a range of temperatureswill produce a family of curves. The maximum and minimum predictedoperating temperatures will produce two limiting curves. For thesecurves the wavelengths are selected such that at those wavelengths thechange in relative intensity from one curve to the other is the same,and hence it will be the same for the family of curves over the fulltemperature range. If the curves are of an appropriate shape it might bepossible to select more than one pair of wavelengths having the desiredproperty of equal changes in relative intensity at both of thewavelengths.

In a two LED system, a relative intensity versus wavelength plot over arange of temperatures will again produce a family of curves. Limitingcases also exist at the maximum and minimum operating temperatures.Again, the selected wavelength for the operation of the LED could be atpoints at which the two limiting curves intersect and hence the familyof curves intersect.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is an illustration of a typical spectral variation of the outputcharacteristic with temperature of a surface emitting LED;

FIG. 2 illustrates the variation of relative intensity with temperatureof an LED output spectrum;

FIG. 3 illustrates the selection of frequencies in accordance with thepresent invention on the basis of a relative intensity versus wavelengthrelationship obtained for a particular LED device;

FIG. 4 illustrates a relative intensity versus wavelength relationshipof a different form from that shown in FIG. 3 again showing theselection of frequencies in accordance with the present invention; and

FIG. 5 is a block diagram illustrating the first stage in signalprocessing for an LED having the characteristics illustrated :n FIG. 4.

Referring to FIG. 1 this shows the large variations in the spectrum ofthe output characteristic of a surface emitting LED with temperature.The left-most curve is that obtained at 0° C., the central curve is thatobtained at 30° C., and the right-most curve is that obtained at 60° C.Clearly if one was to select frequencies of 840 NM and 890 NM as thefrequencies for a dual wavelength sensor system the relative intensitiesof these two frequencies would change radically as the LED temperatureincreased. It is essential for some method of compensation to beprovided for such temperature variations.

FIG. 2 illustrates four curves each illustrating the relative intensityversus wavelength for a respective temperature in one LED device.Different LED devices will have different characteristics. In theillustrated example, it will be seen that all four curves come togetherat a wavelength of approximately 925 NM and that the four curves aresubstantially parallel in the range roughly between 850 NM and 900 NM.Such a characteristic is one example of a characteristic which makes itpossible to automatically compensate for temperature variations in theoptical source of an embodiment of the present invention.

In a first embodiment, the system would use a single LED source toprovide both the measurement and reference signals. As illustrated inFIG. 3 which shows two curves each corresponding to a respectivetemperature for the LED a first (measurement) wavelength λ₁ would beselected and a second (reference) wavelength λ₂ would be selected. Insuch a system any variations in temperature within the range indicatedby the two curves would not substantially affect the intensity of onesignal relative to the other. Thus the system could be operated withconfidence without regard to optical source temperatures.

As an alternative, if two LED sources were used, the wavelength λ₃ couldbe selected for one of the reference and measurement signals. Given thatthe two curves intersect at that wavelength changes in relativeintensity resulting from temperature variations would be negligible.Providing the other signal was produced by a different optical sourcehaving a similar shaped relative intensity versus wavelengthcharacteristic intersecting at a different wavelength again the systemcould be operated without reference to the optical source temperature.

FIG. 4 illustrates the relative intensity versus wavelengthcharacteristic for a different LED source to those illustrated in FIGS.2 and 3. The two curves correspond to the relative intensity at a first(maximum expected) temperature and a second (minimum expected)temperature. It will be seen that the wavelengths λ₁ and λ₂ could beselected to provide changes in relative intensity of equal magnitude butopposite sign. In such an arrangement the receiver circuitry would beadapted to take account of the opposite sign of the changes in relativeintensity but otherwise would operate in exactly the same manner as thesingle LED source embodiment described with reference to FIG. 3.

FIG. 5 illustrates a first stage signal processor for processingmeasurement and reference signals derived from a single LED atwavelengths λ₁ and λ₂, the LED having the characteristic illustrated inFIG. 4. The two signals S1 and S2 are applied to respective differentialamplifiers 1, 2 arranged as shown such that the outputs respond totemperature similarly. Amplifier 1 has a grounded input, amplifier 2 areference input Vref which is greater than the maximum S2 signal. Afurther amplifier 3 provides a referenced output to a signal scalingcircuit 4.

Thus the present invention provides an extremely simple but elegantsolution to the problem of temperature compensation in dual wavelengthoptical sensor systems. To put the invention into effect it is merelynecessary to derive information of the type illustrated in FIGS. 2, 3and 4 and select appropriate frequencies by the use of simple band passfilters. Such filters could be positioned adjacent the optical source,the sensor or at the receiver, or at any point in the optical linktherebetween.

We claim:
 1. An optical fibre sensor system comprising a transmitter, areceiver and a sensor interconnected by an optical link, wherein thetransmitter comprises an optical source arranged to transmit to thesensor via the optical link two optical signals at differentwavelengths, the sensor is arranged to transmit to the receiver via theoptical link measurement and reference signals, the measurement signalbeing derived by modulating the intensity of one of the two opticalsignals in dependence upon a parameter monitored by the sensor and thereference signal having an intensity which has a predeterminedrelationship with the intensity of the other optical signal, and thereceiver is arranged to compare the intensities of the measurement andreference signals to derive a measure of the monitored parameter,characterised in that the two wavelengths are selected such that changesin the intensities of the optical signals resulting from variations inthe temperature of the optical source are minimised or are ofsubstantially equal magnitude for both the optical signals.
 2. A sensorsystem according to claim 1, wherein the transmitter comprises a singleLED and filters are provided to select from the LED output the twowavelengths.
 3. A sensor system according to claim 1, wherein thetransmitter comprises a pair of LEDs each associated with a respectivefilter to provide the two wavelengths.
 4. A sensor system according toclaim 2, wherein the said frequencies are selected such that, if twocurves are produced by plotting the relative intensity versus wavelengthof the source at maximum and minimum predicted operating temperatures,the change in relative intensity from one curve to the other at theselected frequencies is the same.
 5. A sensor according to claim 4,where the magnitude of the intensity changes with respect to temperatureare the same but of opposite sign.
 6. A sensor system according to claim3, wherein for each LED, the respective frequency is selected such that,if a relative intensity versus wavelength plot at maximum and minimumoperating temperatures is produced, the two curves intersect at theselected frequency.